1. Field of the Invention
The present invention relates to an insulating paste, and particularly relates to an insulating paste containing glass powder which is suitably used for forming an electrically insulating layer for high-frequency circuits.
2. Description of the Related Art
With trends towards high density and high speed in high-frequency electronic devices, electrically insulating materials which are associated with high-frequency circuits in these electronic devices are required to have a high dielectric constant and a high Q value. Such electrically insulating materials are used for forming electrically insulating layers which isolate two or more electrodes or transmission lines in, for example, high-frequency circuit substrates or electronic parts for high-frequency circuits.
Holes for electrically connecting upper and lower electrodes or transmission lines through conductive members, that is, via-holes, are often provided in the electrically insulating layer. In the formation of the electrically insulating layer having via-holes, for example, an insulating paste containing glass powder is used. The electrically insulating layer having the via-holes is formed with the insulating paste by, for example, screen printing, and the insulating paste is dried and then baked.
The formation of an electrically insulating layer having fine via-holes based on a photolithographic process using a photosensitive insulating resin such as a photosensitive polyimide resin is also known.
Using an insulating paste which is obtained by dispersing glass powder into a photosetting-type organic vehicle, application of the technology for forming an insulating member by a photolithographic process disclosed in, for example, Japanese Patent Laid-Open Nos. 9-110466 and 8-50811 may be possible in order to form an electrically insulating layer having fine via-holes in electronic parts for circuits which require multiple wiring.
Among the above-described conventional technologies, it is difficult in the printing method to form an electrically insulating layer having fine via-holes having a diameter of, for example, less than about 150 .mu.m because of printing feathering due to paste viscosity and printing resolution due to the printing block. Thus, the printing method cannot deal with recent trends towards miniaturization and high-density of circuit substrates and electronic parts for circuits.
According to the method for forming an electrically insulating layer based on a photolithographic process using a photosensitive insulating resin, although fine via-holes having a diameter of, for example, less than 150 .mu.m can be formed, the resulting electrically insulating layer has disadvantages in durability at high temperature and plating characteristics in an acidic plating solution.
On the other hand, in the insulating materials described in Japanese Patent Laid-Open Nos. 9-110466 and 8-50811 which are composed of glass powder dispersed into a photosetting-type organic vehicle and can form an electrically insulating layer having fine via-holes based on a photolithographic process, the contained glass has a relatively low softening point. When an Ag-based material is used in electrodes or transmission lines connecting to the resulting electrically insulating layer, diffusion of Ag into the electrically insulating layer is significantly noticeable and thus the formation of the electrically insulating layer having high reliability is difficult.